A fire hydrant (or colloqually in the US, fire plug) is a
source of water provided by most metropolitan communities to enable firefighters
to tap into the water supply to assist in extinguishing a fire. It enables
them to draw water from the municipal water supply. To prevent casual use
or misuse, the hydrant requires special tools to be opened.

A hose is attached to the hydrant, then the valve is opened to provide
a powerful source of water, 50 lb/in² (350 kPa) in some areas, more
in others, depending on various factors including the size and location
of the attached water main. This hose can be further attached to a fire
engine, which can then boost the pressure and possibly split it into multiple
streams. Care should be taken not to open or close a fire hydrant too
quickly, as this can create water hammer which can damage pipes and equipment.

Some US communities provide sprinkler heads to enable residents to use
the hydrants to cool off during hot weather. Due to the presence of standing
water inside the hydrant for weeks and months at a time, it is necessary
to drain them on occasion to prevent bacteria from seeping back into the
drinking water system serving the hydrant.

In most US areas, contractors who need temporary water may be permitted
to use hydrants subject to attaching a meter and a clapper valve to prevent
backflow into the hydrant. Additionally, residents who wish to use the
hydrant to fill their in-ground swimming pool are commonly permitted to
do so provided they pay for the water and agree to allow firefighters
to draft from their pool in the case of an emergency.

Due to their constant presence and utilitarian nature, some cities and
towns allow fire hydrants to be painted in various colors and/or as whimsical
characters. The most common colors are red and yellow. In other places,
such as in Ottawa, hydrant colors communicate different messages to firefighters;
for example, if the inside of the hydrant is corroded so much that the
interior diameter is too narrow for good pressure, it will be painted
in a specific scheme to indicate to firefighters to move on to the next
one.

It is illegal, in many jurisdictions (but legal in others such as the
UK), to park a car in front of or within a certain distance of a fire
hydrant. The distances are commonly 10-15 ft, or 3-5 m. Especially in
cities, this is commonly ignored, and causes a headache to firefighters
who need to connect to the hydrant. In the movie Backdraft, a firefighter
faced with this problem smashes the windows in the car and passes the
firehose through the car to the fire hydrant. Whether this scene was inspired
by a true incident or the movie inspired some firefighters to use this
technique, it has happened in actual situations.